function  [x1,y1]=numcross_ben(x,y,Pc)
if  rand < Pc
    %lam1=0.4;
    lam1=rand;
    x1 =lam1* x(:,1:20) + (1-lam1) * y(:,21:30);
    y1 =lam1* y(:,1:20) + (1-lam1) * x(:,21:30);
else
    x1 =x;
    y1 =y;
end

